Induction furnace for heating and temperature homogenization in hot-rolling of thin steel strips

ABSTRACT

PCT No. PCT/IT90/00053 Sec. 371 Date Mar. 14, 1991 Sec. 102(e) Date Mar. 14, 1991 PCT Filed May 17, 1990 PCT Pub. No. WO90/14742 PCT Pub. Date Nov. 29, 1990.An induction furnace is described which is capable to heat a thin strip from a continuous casting apparatus throughout the furnace itself after a previous rolling, at a homogeneous temperature adapted to the subsequent rolling steps. The induction furnace of the present invention comprises, in a known manner, an array of toroidal coils being fed at M.F., with intermediate feeding rollers having reduced size along the strip feed direction with respect to the known coils so that the rollers are less spaced from each other. Flux concentrators are also provided, being distributed preferably in pairs on the inductor and mounted transversely movable with respect to the strip forward movement and possibly also in a plane perpendicular thereto.

The present invention relates to an induction furnace for heating andtemperature homogenization in hot-rolling of thin steel strips producedby continuous casting and already subjected to a preliminary rollingstep.

It is generally known to use M.F. induction furnaces in steel industry.These furnaces are used, although not widely, as an alternative to thegas furnaces for heating of the slabs having a thickness of more thanabout 100 mm, whereas the use with strips having a thickness of lessthan about 30 mm is limited to a thermal treatment of the edges in orderto oppose the natural cooling which, being greater than in the centralzones, causes such a temperature decrease as to produce possible cracksat the strip edges. However this induction treatment of the edges isaccomplished by means of C-shaped inductors which enclose, like slidingshoes, only the fringe areas of the strip during the forward movementand certainly cannot be considered true and actual induction furnaces.

On the contrary the induction furnaces for slabs comprise a row ofinducing coils having a substantially toroidal shape, within which theslab is caused to move forward in sequence, between one coil and theother there being provided driven and/or idle rollers for feeding saidslab. These coils have a size that in the direction of movement are inthe order of about 900 mm.

On the other hand plants are provided at present for the continuousproduction of steel strip by the continuous casting method in order tohave as final product the so-called "coil" or steel plates cut atprefixed lengths, wherein it is required that strips in an intermediatestep of hot-rolling are heated with contemporaneous temperaturehomogenization in induction furnaces for reaching the requiredtemperature in the final steps of rolling. In this respect referenceshould be made to the Italian Patent Application No. 20752 A/88, the PCTApplication DE-88-00628 and German Patent Application No. P-3840812.0.

In this respect the adoption of known induction furnaces, already usedfor the slabs, cannot however give the expected results due to the factthat inconveniences would occur, being caused e.g. to the stripstumbling and possible hitting against the coils owing to its lowerrigidity with respect to the slabs having a greater thickness.Furthermore the strip requires for temperature homogenization that incertain zones, possibly not always the same but such as to be localizedat each time, the heating has to be stronger with a greaterconcentration of flux lines. As a matter of fact this is not required bythe slabs, since the temperature is more homogeneous due to the greaterthickness.

Therefore it is an object of the present invention to provide aninduction furnace for steel strips from continuous casting having athickness of less than 30 mm, as they have been already partiallyhot-rolled so as to heat homogeneously the strip up to the requiredtemperature for the subsequent steps of rolling completion withoutfacing the above-mentioned drawbacks.

The induction furnace according to the invention comprises an array ofcoils, each of which is embedded in an inductor unit, separately fed byone or more frequency converter and successively crossed by the stripbeing supported and driven through pairs of rollers between each coiland the subsequent one, the size of each coil in the forward directionof the strip being less than 500 mm, there being also provided fluxconcentrating devices which are distributed in pairs on each inductor,at least at one upper or lower side with respect to the plane defined bythe strip itself.

According to a preferred embodiment of the invention the fluxconcentrating devices are mounted movable in a transverse direction tothe strip and preferably also perpendicularly to the strip itself for abetter distribution of power in the areas where a stronger heating isrequired. These and additional objects, advantages and characteristicsof the induction furnace according to the invention will be clear to thepersons skilled in the art form the following detailed description of apreferred embodiment thereof, given by way of a non-limiting example,with reference to the annexed drawings in which:

FIG. 1 shows a partial, diagrammatic side view of a heating element ofthe furnace according to the present invention; and

FIG. 2 shows a cross-section view taken along line II--II of FIG. 1.

With reference to the drawings, FIG. 1 schematically represents aportion of the furnace according to the invention along the forwarddirection of strip 1, as regards only one heating element of the furnaceitself, comprised of a coil 2 having a substantially rectangularcross-section with rounded edges (better seen in FIG. 2) which isembedded within an inductor 3, as it is surrounded by refractorymaterial. As better shown in FIG. 1 the strip 1 passes through the spacedefined by each coil 2 (the height of which will be not less than themaximum strip thickness which may be expected), as it is driven byrollers 10 positioned between each heating element or coil 2 and thesubsequent one. The rollers 10 can be all motorized or some of them maybe idle.

Through the inductor 3 embedding the coil 2, this is fed on one side bymeans of suitable conductors 6 usually called "bus bars" by a source ofgiven power and frequency, in particular one or more converters (notshown). Advantageously the frequency will be fixed once and for all, andpossibly the feeding power will be varied according to the energyrequired for heating, as a function both of the strip temperatureupstream of the furnace and of its thickness with a finer regulationbeing responsive to the temperature at the outlet of the coil. At theopposite side of the feeding connector 6 there may be provided manifoldsfor the delivery and discharge of the cooling water genericallydesignated 7 in FIG. 2 and connected to the inductor 3.

According to the present invention there are also provided fluxconcentrators 4 formed of packs of magnetic sheet iron for directing theprimary electro-magnetic field flux so as tot concentrate it in adirection substantially parallel to the forward movement of the strip.In the area of strip 1 where the concentrated flux closes its circuit aninduced current is produced, having a higher intensity and therebygreater heating. The flux concentrator have been represented as formingtwo pairs, an upper one and a lower one. The concentrators 4 pertainingto the upper pair correspond to the concentrators 4' of the lower pair,whereby the concentrators are coupled two by two on either side of thestrip 1, as they are co-axial with an axis passing throughout strip 1and coil 2. Flux concentrators 4 and 4' will be normally positioned inthe proximity of the edges of strip 1, just where a greater heatingpower is required.

However preferably, as shown in FIG. 2, the flux concentrating devicesare designed to be movable firstly in the transverse direction both tofollow the dimensional variations of the strip in width and to bepositioned also in correspondence of inner zones which may be at al owetemperature (cold spots), and possible also in a perpendicular directionto the plane defined by strip 1 for abetter positioning in the heightdirection in function not only of the strip thickness but also of thepower to be concentrated.

As shown in the drawings, in particular in FIG. 2, the two pairs of fluxconcentrators 4, 4' are mounted, with each element opposite to theassociated one of the other pair, to an inner nut thread on screws 9operable from the outside by means of control handwheels 5, 5'respectively. The screws 9, 9' will have one half of their length with athread to a direction and the other half with a thread oppositelydirected, so that the movement of the two concentrators of each pairwill be symmetric and self-centering at each operation of the associatehandwheel. Additionally the screws 9 and 9' will be preferably mountedat their central portion to a lifting and lowering device which is alsocontrollable by means of outer handwheels 5a, 5a'. Said handwheels maybe for example fixed respectively to screws 11, 11' being perpendicularto the screws 9, 9', each of them passing through a hole at right angleto the longitudinal axis of the latter.

Of course a completely automatic control of the concentrators 4, 4'positioning may be provided, both in a transverse and in heightdirection. Instead of the operating handwheels, step by step motors willbe used, being interlocked with a regulation and control unit adapted toprocess directly in real time the strip temperature, speed and thicknesssignals as received each time at the furnace inlet. In this way at eachmoment an optimal adjustment of the flux concentrators can be obtainedfor a better efficiency of the induction furnace according to thepresent invention.

EXAMPLE

For experimental purposes an induction furnace plant according to theinvention was installed, having suitable longitudinal size of the coilsand being provided with pairs of flux concentrators in association witheach inductor. The main date of installed power number of inductors,frequency and size of the coil port in height, as well as of the resultsobtained with reference to the consumptions according to the stripcross-section and its feedings speed, efficiency etc. are listed in thefollowing table also reporting the corresponding data for an inductionfurnace according to the prior art, namely of the type used for heatingslabs, in which only the size of coils in the forward direction of thestrip have been reduced without however the presence of the fluxconcentrators. As regards the consumptions it should be noted that theyhave been detected at the highest speed of the strip for temperatureincreases (ΔT) of about 150° C. and, at the lowest speed for ΔT of about300° C.

                  TABLE                                                           ______________________________________                                                         With concent-                                                                 rators accord-                                                                ing to the                                                                             Without                                                              invention                                                                              concentrators                                       ______________________________________                                        Installed power (MW)                                                                             16          21                                             Frequency (kHZ)     6          10                                             Number of inductors                                                                              20          26                                             Inductor port      65          55                                             (height in mm)                                                                Consumptions                                                                  (Kwh/t - ΔT = 150° C.-300° C.)                            15 × 1050 mm v = 0,19  m/sec                                                               127        168                                             15 × 1050 mm v = 0,237 m/sec                                                               79         127                                             25 × 1330 mm v = 0,086 m/sec                                                               100        169                                             25 × 1330 mm v = 0,143 m/sec                                                               42          96                                             20 × 1330 mm v = 0,11  m/sec                                                               110        177                                             20 × 1330 mm v = 0,178 m/sec                                                               50         106                                             Efficiency of inductors                                                       25 × 1330     80%         79%                                           15 × 1330     79%         67%                                           ______________________________________                                    

As it appears from the data given with the furnace plant according tothe invention, not only a reduced installed power can be foreseen butlower consumptions of energy are obtained with the same cross-sectionand speed of the strip, as well as better efficiencies of the inductors.

Possible additions/or modifications can be made by those skilled in theart to the above described and illustrated embodiment of the inductionfurnace according to the present invention without departing from thescope of the invention itself. On the other hand no limitation will beexpected as to the number of coils or heating elements in succession toeach other which form the furnace.

What is claimed is:
 1. An induction furnace for heating a steel strip(1) having a thickness lower than 30 mm and for rendering homogeneousthe temperature up to a value required for the subsequent steps of hotrolling, said induction furnace comprising:an array of coils (2) eachhaving along its length a rectangular cross section and a slotcorresponding to the width and depth of the steel strip (1), each ofsaid coils embedded in a refractory material at the inside of aninductor unit (3), each said inductor unit (3) fed by one or morefrequency converters (6), each of said coils (2) being crossed insuccession by the steel strip (1), as it moves through the furnace, thesteel strip (1) being supported and housed to continuously move throughsaid slot in each of said coils (2) by rollers (10), one of said rollers(10) located before and after each of said coils (2) in the direction ofmovement of the steel strip (1), wherein said coils (2) are placed sideby side with the length of each of said coils (2) being perpendicular tothe longitudinal axis of the steel strip (1), wherein a magnetic fieldflux is generated in the plane of the steel strip (10 and parallel toits movement through said slot in each of said coils (2), wherein thewidth of each of said coils (2) in the direction of the steel strip (1)movement is less than 500 mm, and p1 flux concentrator devices (4, 4')distributed in pairs on each said inductor (3) at least on an upper orlower side, with respect to he plane defined by the steel strip (1). 2.A furnace according to claim 1, wherein each of said flux concentrators(4, 4') is mounted substantially co-axial with the correspondingconcentrator of the other pair at the opposite side of strip (1), eachof said flux concentrators (4, 4') being formed of packs of magneticsheet iron.
 3. A furnace according to claim 1, wherein said fluxconcentrators (4, 4') are all positioned stationary in the proximity ofthe side edges of strip (1).
 4. A furnace according to claim 1, whereinsaid flux concentrators (4, 4') are mounted movable in the transversedirection to the strip (1).
 5. A furnace according to claim 1, whereinsaid flux concentrators (4, 4') are movable also in the directionperpendicular to the plane of strip (1).
 6. A furnace according to claim1, further comprising screw devices (9, 9'; 11, 11') for the adjustmentof said flux concentrators, engaged to outer control handwheels (5, 5';5a, 5a').
 7. A furnace according to claim 1 further comprising drivingmeans for the automatic movement of said flux concentrators 4, 4'interlocked with a means for processing data of strip (1) speed,temperature and thickness as sensed by detecting means upstream of thefurnace, as well as of the temperature at the outlet.